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José Carlos Fernández Rodríguez IT/Web Office
XML (Extensible Markup Language) is a simple subset of SGML (Standard Generalized Markup Language) and it is intended as a metalanguage to define markup languages.
HTML is one of the markup languages that can be defined (and is currently being defined) as an application of XML. Other examples are MathML and CML for mathematics and chemical formulae respectively. Extra markup languages will provide extra tags that are not in HTML (in fact may have nothing to do with HTML). A much better control is thus given to authors. Another significant advantage of XML is the ability to mix different markups in a single document, hence an HTML document may include MathML sections.
XML attempts to dissociate structure from presentation, hence it does not define how content should look on the screen or on paper.
On August 18th, the World Wide Web Consortium issued the First Public Draft of XSL 1.0 (Extensible Style Language). This is a very important step in the standardisation process of XML related technologies, because this new style sheet generation will allow developers to produce multiple different views for the same XML document.
This fact can help to solve the main current problem of XML documents: they cannot be displayed by using a normal general browser. So far, there is no standard way to link a style sheet to a document, so browsers are not able to know what to do with the bunch of new tags that are included in the document.
An XSL style sheet is used to convert a set of XML elements to a sequence of the so-called flow objects in the old SGML terminology. These flow objects are an intermediate tier that can be later transformed to different output formats very easily. For example PostScript, RTF and HTML outputs can be generated for the same XML document.
On the other hand, a whole set of XML applications have appeared and are being developed. Three significant examples are CML (Chemical Markup Language), MathML and RDF (Resource Description Framework).
In conclusion, one can see that XML technologies are still evolving, and it seems they are going to be quite important in the future. The Web Office will follow this evolution, but for the time being will make no specific recommendation.